Tuesday, July 26, 2016

PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl

#11,584

One of the great unresolved avian flu mysteries is why - after the largest epizootic outbreak in U.S. history over the winter-spring of 2015 - did the virus fail to return the following fall?

The virus arrived in the Pacific Northwest via migratory birds in November of 2014, and ultimately spread to poultry in 15 states, resulting in billions of dollars in losses.

The expectation was it would appear again last fall with the return of migratory birds, but that didn't happen. And wild bird detections of HPAI since last summer have been practically nil.

To try explain this unexpected turn of events, we've a study appearing in PNAS (alas, behind a pay wall) - with Robert G. Webster of St. Jude Children's Research Hospital as the corresponding author - that concludes that migratory waterfowl are not a reservoir for highly pathogenic avian flu viruses.

This has significance beyond just further defining the ecology of HPAI, as it validates the choice to employ traditional eradication methods - quarantine, culling and disinfection - rather than rushing to use a poultry vaccine.

One of the major unresolved questions in influenza A virus (IAV) ecology is exemplified by the apparent disappearance of highly pathogenic (HP) H5N1, H5N2, and H5N8 (H5Nx) viruses containing the Eurasian hemagglutinin 2.3.4.4 clade from wild bird populations in North America.

The introduction of Eurasian lineage HP H5 clade 2.3.4.4 H5N8 IAV and subsequent reassortment with low-pathogenic H?N2 and H?N1 North American wild bird-origin IAVs in late 2014 resulted in widespread HP H5Nx IAV infections and outbreaks in poultry and wild birds across two-thirds of North America starting in November 2014 and continuing through June 2015.

Although the stamping out strategies adopted by the poultry industry and animal health authorities in Canada and the United States—which included culling, quarantining, increased biosecurity, and abstention from vaccine use—were successful in eradicating the HP H5Nx viruses from poultry, these activities do not explain the apparent disappearance of these viruses from migratory waterfowl.

Here we examine current and historical aquatic bird IAV surveillance and outbreaks of HP H5Nx in poultry in the United States and Canada, providing additional evidence of unresolved mechanisms that restrict the emergence and perpetuation of HP avian influenza viruses in these natural reservoirs.

The H5 avian influenza A virus that devastated North American poultry farms in 2014-15 was initially spread by migratory waterfowl, but evidence suggests such highly pathogenic flu viruses do not persist in wild birds. St. Jude Children's Research Hospital led the research, which appears online this week in the Proceedings of the National Academy of Sciences.

While wild ducks and other aquatic birds are known to be natural hosts for low pathogenic flu viruses associated with milder symptoms, the results of this study indicate that is not the case with the highly pathogenic flu viruses that are associated with more severe illness. The research suggests that wild ducks and other aquatic birds are not an ongoing source of highly pathogenic flu infection in domestic poultry.

"The findings provide a scientific basis for the decision by officials to use culling and quarantines to stop the 2014-15 outbreak in domestic poultry," said corresponding author Robert Webster, Ph.D., an emeritus member of the St. Jude Department of Infectious Diseases. "Now, research is needed to identify the mechanism that has evolved in these wild birds to disrupt the perpetuation of highly pathogenic influenza."

In this study, researchers analyzed throat swabs and other biological samples taken from 22,892 wild ducks and other aquatic birds collected before, during and after a 2014-15 H5 flu outbreak in poultry. The outbreak has been linked to a highly pathogenic H5N8 influenza A virus spread from Asia to North America by migratory waterfowl. The H5N8 virus reasserted, or mixed genes, with other influenza viruses in North American waterfowl and went on to trigger 248 flu outbreaks in commercial and backyard turkey and chicken farms in the U.S. and Canada at a cost of nearly $5 billion.

Officials worked to end the outbreaks by quarantining and eliminating infected poultry. The last confirmed case occurred in June 2015. Officials worried that the highly pathogenic virus would be re-introduced into poultry farms by migratory aquatic birds carrying the virus. But none of the migratory birds included in this analysis were infected with a highly pathogenic flu virus. The sampling was conducted in Canada, the Mississippi flyway and along the U.S. Atlantic coast by David Stallknecht and Rebecca Poulson of The University of Georgia and Richard Slemons, Andrew Bowman and Jacqueline Nolting from the The Ohio State University in conjunction with Scott Krauss and James Knowles of St. Jude. The sampling was done as part of the federally funded Centers of Excellence for Influenza Research and Surveillance.

Such viruses have not been identified in any of the more than 100,000 wild birds tested since the flu surveillance sampling began 43 years ago, Webster said. "Existing immunity in wild birds is one of the possible explanations that may explain why highly pathogenic influenza A viruses do not become established in wild bird populations," he said. "But a more complete understanding of the mechanisms at work would aid efforts to prevent, control and eradicate these dangerous viruses in poultry in other areas of the world."

Webster added that while there were no reported human cases of influenza caused by the highly pathogenic flu viruses involved in this outbreak, other related H5 viruses have spread to humans with deadly results.